CVE-2026-21903 Junos OS: Subscribing to telemetry sensors at scale causes all FPCs to crash

High 7.1 Published 15 Jan 2026 Updated 15 Jan 2026

Authenticated low-privilege attackers can cause a denial of service by triggering a stack-based buffer overflow in the Packet Forwarding Engine via telemetry subscriptions.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is CVE-2026-21903?

CVE-2026-21903 is a high-severity memory corruption vulnerability in Juniper Junos, published on 15 Jan 2026. Any signed-in user can exploit it over the network, with no action from the victim.

Official description

A Stack-based Buffer Overflow vulnerability in the Packet Forwarding Engine (pfe) of Juniper Networks Junos OS allows a network-based attacker, authenticated with low privileges to cause a Denial-of-Service (DoS).

Subscribing to telemetry sensors at scale causes all FPC connections to drop, resulting in an FPC crash and restart. The issue was not seen when YANG packages for the specific sensors were installed.

This issue affects Junos OS:

* all versions before 22.4R3-S7, * 23.2 version before 23.2R2-S4, * 23.4 versions before 23.4R2.

Weakness: CWE-121

Which versions are affected by CVE-2026-21903?

It affects 1 product.

Products include Juniper Junos.

How do I fix CVE-2026-21903?

No fix has been published for CVE-2026-21903 yet. Limit exposure of the affected software until one is available.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

Is CVE-2026-21903 being exploited?

There is no known exploitation of CVE-2026-21903 in the wild and no public exploit code.

All Juniper Junos vulnerabilities

Sources

Also known as GHSA-h3c8-3472-7v7g.